Reducing viewport framerate
I don't think that is an option, but here are some general tips.
In Rendered View, there's a button in the top right to pause rendering.

In Render Properties there's a Simplify section you can use to cut down several of the most taxing effects without having to make any changes to the objects in the scene.

If the problem is not the scene geometry, but rather the amount of time it takes to run individual samples, then I suggest a render region with Ctrl + B.  You could also use Local View with / to focus on your selection.